This invention relates to apparatus and method for colorimetric analysis. More particularly, the invention relates to an automatic colorimetric analyzer which provides for the complete and efficient mixing of a sample and a reagent used for color development. Known methods and instrumentation provide for the mixing of a liquid reagent and a liquid sample by means of simultaneous cocurrent flow of the liquids through a common passageway. In addition, U.S. Pat. No. 2,362,278 shows continuous sampling at or above atmospheric pressure in combination with a turbidimetric device. U.S. Pat. Nos. 3,028,225 and 3,440,016 show continuous sampling and color development at atmospheric pressure, in combination with a colorimetric device. French Pat. No. 1,229,095 shows continuous sampling and color development, in combination with a chromatographic column and a colorimetric device.